Process for the desiccation of labile aqueous materials



,UNITVEDQST'IATES PATENT,oFFlCEJ v PROCESS FOR-THE DESICCA'llION 0F LABILE AQUEOUS MATERIALS; ,1

V Earl W. Flosdo rii Forest Grove, Paala'ss'ignor" to WP il -Cryochem Corporation, Baltimore,

1'] mm i fMary anq i V ,VNOVDIaWiH-g- Application August 5, 19 47, 4 Serial No. 766,475,; M

This invention relatesto improvements inthe desiccation of aqueous materials fromthe frozen state, and more particularly to improvements by which the rate of desiccation may be materially increased by increase of the rate at which the last .desiccated is'essentially limited to the rate at which it is possible to. supply heat to the frozen portions of th water present in the aqueous material are removed.

Desiccation of various materials, such as biologicals," foodstuffs and labile chemicals are known. In general, they involve freezing the material andsubliming the ice therefrom by the application of a high vacuum until substantially all of the water is removed therefrom, such that the product has a moisture content which may rangefrom about .5'to about 25%, and subsequently raising'the temperature of the material to room temperature or even to higher temperatures such as 50 or 605C. to remove the last traces of the water and to reduce the water content of the material to the lower level required for stability on storage, nearly always below 2%, usually 1% or 0.5% or less.

I 'The process is carried out at a very low pressure or high vacuum, usually of the order of 50 to 700 microrisgalthough certain materials, under certain conditions, may be desiccated at higher-pressures, for example, 2 to 3 mm. The requirement as to pressure is that it be sumciently low, below the vapor tension of the frozen material, so" that the frozen water sublimes at,

a rate sufficiently high to keep the material frozen, or at least in a congealed, i. e., plastic, state. 1

In carryingout this operation, it is necessary that the latent heat of sublimation of the water be supplied. In most of the operations used today, the process is carried out by placing the material to be dried in a vacuum chamber provided with means for heating it such as heated shelves, heating jackets, or both, radiant heatthe speed otthe drying operation during the 50 "111i ::ld d

material without either '(1) causing any part of it toliquefy, or (2) overheating any portion of it which may be dried.

In many normal. operations, involving the'desiccation of a wide range of biologicals' and other materials, it hasfll'oeen found that it is necessary, to insure the proper low moisture content of the final product, to continuethe application of the high vacuum for' a substantial'period of time afterv the temperature of the frozen material rises' above 0 0., representing the point at which the ice has all been'sublimed and the cooling effect of thesublimation is insufiicient to keep the temperature Tof the materialv down, without anylincreas'einthe rate at which heat is supplied by the heating medium. I

The present invention provides improvements which radically shorten the time period required to removethe water whichremains in the product undergoing desiccation after the ice has been sublimed and what is required is the removal of the residual water. In accordance with the present invention the. initial removal of water, that is, the sublimation of the ice, is carried out in the usual way with the material in trays or other containers in a vacuum chamber and with the heat being supplied by circulating a heating medium through the jacket of the chamber and its shelves, or by electricallyjheating'the shelves,

' 0, further heatis supplied by subjecting the material to .a' high frequency-electric field, which causes the generation of heatwithin the material itself, and, radically increasesthe rate at which the last portionsj of water are evaporated. V In the process of the present application, the

amount of heat required to be supplied dielectrically is merely that required to cause the evaporation of the water .reinaining' after the sublimation of ice is finished corresponding to the hieat'of evaporation offfrom 5 to 25% of water I based ion the final solids, that is, approximately l/ th to g /i'ip'ouna :q'r water peround of dry solid. .l'ff' a. at:ta emi p a tic 'Q ii,

vacu m chamherimay vhe J cons d a e a d to th se e in accordance with the present invention most of the heat is supplied by inexpensive sources of heat such as circulating hot water or'steam or the like and only a small proportion of the required heat is supplied dielectrically, the present invention has the advantage that the electric field may operate at a much lower frequency than it is possible to operate in the process of the Bradbury application. Thus, that application points out that. during the desiccation operation when ice is present it is necessaryto have a vacuum sufficiently' high to cause the sublimation to proceed at a rate which, despite the supply of heat, keeps the material cold enough to prevent it from liquefying, i. e., a vacuum of .theor'der of 50 to 750 microns. At such low pressures, .high frequencies in excess of 200 megacycles. arerequired to prevent ionization .or iarcing withinthe vacuum chamber. In contrast with this. in'the practice of the present invention the electric field is not used in that part of the operation" where sublimation of ice occurs and where-it is requires the placing of plates within the chamber, insulated from each other, in such position that the frozen material is between them, the plates being connected to a suitable source of high frequency current. Upon supplying the high frequency current to these plates the high frequency electric field which serves to generate the heat within the material as directed above is established.

Theoperation, in its initial stages then proceedsin accordance with'the usualpractice, the heat required being supplied by circulating a heating medium through the shelves, heating them electrically, by radiant heat or in other jways. 11 At that stage of the operation at which theice is all sublimed, which is readily deternecessary to keep the temperature of the -ma- "ticejofj the @pr sent" process, :oielectric heating is not us d .un fiiihe ieei' as' en'sub ime jan t temperature of the material may beraised above -.W hout, c1amaee to "Consequently in't e pro ss th 'pr su e'W hin t at h st es t on ra o iai whic die ct e t n u e. when resultrthata cins a ionization. are inhibit and much lower freq eni i yfor e am e; rom 1 100120 meeacy a e s d-i This; i a van a eous from th standpoint of availability of oscillating tubes suitable jfor use in supplyingthe necessary power.

.Thus, the pr ssures ma nta n -din the acuum cham er d hsj h p ri d t a the d ectric field is used forsupplyingheat may v,beofithe order of to 3",.' "mer ury. as di in uished from th much" highe vacuum i sedin e'i ifiall'sfflagQs. o t op ration when ice, ispresent audit i in gessar 'top even its iiquei etion by i sur n an a equatel ra idsublimat on.

The procejssma'ybe carried out in anyconvenient apnaratus'.i.a d requ res little modification. of the "apparatus which is currently ,used.

nneitype which isjmore or less widely usedponsists of alargechamber.capableof beingsealed with a vacuu'rnjtightgsealand provided with a large outletf leading ito ;a cold condenser and be placed .on' thelslielv'es The mater ia1;is frozen inthe trays lor containers leither b fore placin them in' the'ichamber .or after'placing them in V the chamber .andby thejuseof .a refrigerant. or

b autorefiieeration induced. by the application 7 all as is wellknown I athe. art. ,'When*the...frozen mate a1 "is in place the 'Vacllum chamber .is

se an Z nc ivacu im Pump s tarted- "Th acuum. Pu r mov "t manent :ea

dike -water a or. which. s

from the isy ed i "thi old o den e generated is con -which,.of 'coursefis maintained at "a'temperature lower than that of the frozen material are be d cat d .170 nos i t is we k o a p tus for practicing "the present invention merely mined by themarked upturn in the temperature ,of' theimaterial undergoing desiccation as traced by suitably positioned thermocouples or the like, 29'

and about the time when the temperature of the material-rises above its freezing point, the high frequency electric field is turned on; causing generation of heat within the material itself to rapidly remove the'small proportion of water which is still present. The onlyheat generated by "the high frequency electric field is in those u portions of the material which still contain some water. Noheat is generated in' any portions 'whichmavbe dry. "The possibility'of overheating'thematerial'is therefore substantially vavoided because no portion will be heated ,h'otter'than that temperature at which the vapor pressure .of the .water present corresponds to the pressure within .the chamber. As soon as the water is substantially removed the heating'effect-at that part of-the material ceases. 'Inthis way, instead of' requiring along period-of time'to insure the removal of thelastportions of water ,down to :the :required :low content lrequired, which :has

rfrequently'beenfrom 112 to 36 hours ormore,

depending upon the particular material being dried-andthe quantity being dried, the last porpermitting a radical increase in the capacity of the apparatus. Unless. an extremely highfrequency'electric field, such, for example, ,as .one operating. aboveJZOO megacycles is used, it is .nec- ,essary toincreasethepressure. in the-chamber at the timethe. electric iieldis turned on to prevent ionization and. arcing. Thus, if the-field has -a frequency about-10. 110 '20 megacycles the pressure may be increased to about 2" or :3' 0f 'mercury. Atfith s pr ssure-th material will rapidly dry 'whenits temperature is raised bythedielectiio heat generated without bringing the temperature j'of :the; material "being :dried to an :excessive 'level.

:I In zthezprocess of desiccatin'g' labile :aqueous of. sublimationito the-materialgwhile' ice is present :by :external :heating uneans, and subsequently .supplyinglheatatoithematerial, afterallofthe 'ice has :been :sublirned; by dielectricrheating by subjecting it ;to :a l -high frequency ;electric tfield,xraising riitsxtemperature :above 30 10. and-facilitating theremoyal :oi .the remaining water.

EARL w. r'nosnoien.

I (References on "following -page") The following references are of record in the V file of this patent:

I OTHER REFERENCES "Drying and Laminating by Radio Frequency, by George F. Russell, The Timberman. August 1944, pages 38 to 42.

REFERENCES orrmi UNITED TA PATENTS s TES 5 "All-Electronic Penicillin Drying System, Number ,Name Date Chemical and Engineering News, p. 1450, No. 16,

2,292,447 Irwln Aug. 11, 19 2 l 23 Aug. 25I 1945 2307302 Relchel 1943 Radio-Frequency Dehydration or Penicillin 2353986 Barr July 1944 Solution," by Brown et al., reprinted Feb. 1946, 2396561 Flosdorf Mann 1946 10 RCA Laboratories, Radio Corporation of Amer- 2,427,786 Hoyler Sept. 23, 1947 1C9" 2442'114 Brown May 1948 "The Chemical A e" article b G. Colman 2,454,370 a bi n Nov.2s.19 a Green, 9, 1946, i 

